Heat exchanger system



Oct. 30, 1'956 s. B. scHAPlRo ET AL 2,768,934

HEAT EXCHNGER SYSTEM 2 shetS-sheet 1 Filed Feb. 26, '1952 Sylvan B.Schop/ro BY Mark 6. Hopkins A TTOR/VEY Oct. 30, 1956 s. B. scHAPlRo ETAL 2,768,934

HEAT EXCHANGER SYSTEM Filed Feb. 26, 1952' l 2 Sheets-Sheet 2 ATTORNEYUnited States ima'r EXCHANGER SYSTEM Application February 26, 1952,Serial No. 273,431

3 Claims. (Cl. 196-52) This invention relates to an improved heatexchanger system and it pertains more particularly to an improved methodand means for avoiding the build-up of deposits on surfaces of heatexchangers, such for example as are employed for preheating chargingstock to fluid catalytic cracking processes with hot slurry oil producedin such processes.

In uid catalytic cracking systems employing solid catalyst of smallparticle size, some catalyst particles are carried overhead from thereactor to a point near the base of a main fractionator or scrubbertower wherein such solid particles are scrubbed out of the partiallycondensed product stream by heavy gas oil components thereof. The slurryof solid catalyst particles leaves the base of the scrubbing zone of thefractionator at a temperature of about 600 to 650 F. This hot slurry isthen cooled by heat exchange with incoming charging stock, a part of thecooled slurry being returned to a higher level in the fractionator (theupper part of the scrubbing zone) for effecting partial condensation ofreactor efiiuent and removing solids therefrom, while another portion ofthe cooled slurry is sent to a settling zone in order to remove as muchas possible of the heavy gas oil from the solids before the solids arereturned to the catalytic cracking system. The heavy gas oil portion ofthe product, which is also called slurry oil, is undesirable as acomponent of Vthe cracking charging stock because it consistspredominantly of polycyclic aromatics which tend to form excessivelylarge amounts of coke on the catalyst when contacted therewith undercracking conditions.

VEver since the advent of the fluid catalytic cracking process in about1941, the fouling of the fresh feed side of the heat exchangers hasconstituted a costly and vexatious problem. It has been the practice topass the fresh feed through the shell side of the exchanger and theslurry through the tube side thereof since the slurry is more viscousthan the fresh feed and because of the belief of skilled operators thatsolids could riot be satisfactorily handled on the shell side because oftheir tendency to settle out in certain portions of the shell and erodeother portions of the exchanger. Heretofore it has been the universalpractice to clean periodically the shell side of the exchangers by meansof a caustic or steam treatment but such cleaning operations have notreally solved the problem; not only are these treatments troublesome andexpensive, but they fail to remove all of the deposits and eachexchanger gets less and less eflicient as the run proceeds despite thefrequent cleaning operations. An object of our invention is to avoid thenecessity of such cleaning operations, to avoid the continuous build-upof deposits, and to enable heat exchangers to operate at maximumefficiency throughout an entire run length of at least a year or more.On an economic basis, our object is to effect savings in operating costsof as much as $200,000 to $300,000 a year on each catalytic crackingunit of approximately 30,000 barrels per day capacity.

While the precise composition of the deposits cannot rice be determined,it appears that such deposits are at least partially due to oxygenabsorbed in the gas oil feed stock; considerable reduction yin depositsmay be elected by simply stripping the incoming charging stock with fuelgas to eliminate, as far as possible, any oxygen which might bedissolved in said stock. Such stripping, however, does not completelyavoid fouling difficulties and although it may be employed as a part ofour technique, our invention may make it possible to avoid the expenseof stripping oxygen from the original charging stock.

ln practicing our invention, we provide an arrangement of valves andpiping in the heat exchanger system so that the materials iiowingthrough the tube side and shell side of the exchanger, respectively, maybe periodically reversed. The combined solvent action of the slurry oiland scouring action of the solids containedtherein will substantiallyentirely remove deposits which accumulate on the fresh feed side of theexchanger after a short period of operation so that by periodicallyreversing materials llowing through the separate sides of the exchangerat intervals of about l to 20 days, the heat exchangers may operate atsubstantially design heat transfer coecient throughout the entire runlife. ln new installations, it may be desirable to modify `somewhat thedesign of the heat exchangers so that the resistance to ow will be morenearly uniform on the shell side and the tube side, respectively, whencharging stock and slurry oil are alternately passed therethrough;however, our invention is applicable to shell and tube type heatexchangers currently employed iii catalytic cracking units and althougha small amount of solids may settle out in corners where baies join theshell, there is no plugging problem and passage of the slurry oilthrough the shell side is remarkably eifective in cleaning depositstherefrom. Usually, we prefer to 0perate with the fresh feed passingthrough the shell side of the exchangers most of the time; a relativelyshort operation of about 2 hours to 6 days, with the slurry passingthrough the shell side, is sumcient to remove accumulated deposits.

some cleaning action on the shell side is obtainable by returning thesolids from the slurry (after the bulk of the slurry oil has beenseparated therefrom by settling and decantation) to the fresh feedbefore the fresh feed enters the heat exchanger system. In this methodof operation, however, there is constant ow of solids through the shellside of the exchanger and hence a greater tendency toward undue erosiontherein. Furthermore, this method of operation does not utilize thesolvent action of the slurry oil in helping to remove any solids whichmight accumulate.

The invention will be more clearly understood from the followingdetailed description of a specific example thereof as applied to a30,000 barrel per day lluid catalytic cracking unit. Since units of thistype are well known to those skilled in the art, this description willbe limited to the fresh` feed preheat section thereof as illustrated inthe accompanying drawings wherein Figure l is a schematic ow diagramillustrating the initial fresh feed stripping heat exchanger system andpreheat furnace, and

Figure 2 is a more detailed diagram illustrating the structure of eachheat exchanger.

Referring to Figure 1, about 30,000 barrels per day of gas oil chargingstock is introduced through line 10 to stripping tower ll which isoperated at about 30 p. s. i. g. and at a temperature of about F. About22,000 cubic feet per hour of fuel gas is introduced into the stripperthrough line 12, the stripper overhead being Withdrawn through line 13to a low pressure separator. The purpose of the stripping operation isto remove dissolved oxygen from the incoming charging stock and .valves17a and 17a `are open.

vthe base of tower 11 passes by line 14- to pump yl5 and thence by line16 to the exchanger system. As heretofore operated, the charging stockstream was evenly divided with half of it passing through valve 17, theshell -side of exchanger iii, line 19, the shell side of exchangery20,\l ine 21, furnace preheat tube 22 and transfer line l23, catalystbeing picked up from a standpipe into the transfer line and carried bythe preheated vapors into the cracking reactor. Overhead from thereactor was lntroduced in the base of the scrubbing section of afracltionator vwherein the heaviest components of the product werecondensed and'catalyst particles were scrubbed out of the ascendingvapor stream, about 33,00() barrels per day of slurry oil with suspendedsolids being withdrawn from the base of the scrubbing zone at atemperature of -about 600 to 650 F. Half of this slurry was introducedby line 24- to the tube side of exchanger Ztl, thence by line 25 to thetube side of exchanger i8, and was then discharged by line 26, a part ofthe cooled slurry being sent to a settler for removing, by decantation,as much as possible of the slurry oil before returning the solids to thecracking system and another part of the cooled slurry being introducedinto the fractionator at the top of the scrubbing section to condensethe highest boiling :components of the products introduced thereto andeffect further removal of solids. The other half of the charging stockand slurry were handled in a similar manner as designated by likereference characters with 'anf added prime When operated in the mannerhereinabove described, the shell side of exchangers 18 and 20 (also 18and 26') gradually became fouled by deposits to such an extent that theheat transfer coeicient was markedly impaired; it was necessaryperiodically to bypass each exchanger land to clean out the shell sidethereof. The clean-out methods heretofore employed were not onlytime-consuming and expensive but they were not entirely satisfactorybecause cach cleaning operation left a little more deposit than was leftby the previous cleaning so that during the course of the run, more andmore deposits Iaccumulated on the shell side of the exchanger despitefrequent cleaning operations.

In Iaccordance with our invention, additional piping and valves areadded to the heat exchange system and for the purpose of simplicity,reference characters will be applied only to the valves since referenceto the valves will designate any added lines in which these valvesoccur. Valves 17a and l7a are provided to enable the charging stock toenter the tube side of the exchangers through the inlets where slurryoil was previously introduced thereto. Valves 23 and 28 are added tostop slurry flow through lines 25 and 25', respectively, when Valves 23aand 28a are provided for bypassing slurry to the shell side inlets whenvalves 28 and 28 are closed. Valves 29 and 29 are provided to stop flowthrough lines 26 and 26 when charge is introduced thereto through valves17a and ll7a; when valves 29 and 29 are closed, valves 29a and 29a' areopen so that the charge stream may be returned to lines 19 and 19. Whenthe flow of charge is through valves 29a `and 29a', valves Si) and 34Bare closed and material leaving the shell side of the exchanger larepassed through valves 30a and 34M to points in lines 26 and 26 beyondclosed valves 29 and 29'.

From the above description, and referring only to exchanger 18, it willbe seen that by simultaneously closing valves 17 and 23 and openingvalves l7a and 28a the charging stock can be made to enter the tube sideof the exchanger where slurry oil previously entered, and slurry oilmade to enter the shell side where charging stock previously entered. Assoon as gas oil charging stock introduced through valve 17a reachesvalve 29, valve 29 is closed and valve 29a opened while simultaneouslyvalve 30 is closed and valve 30a is opened. rThe system will now beoperating vwith all of the original valves closed and the alternatevalves opened (a, indicating `alternate valves) so that there is areversal of materials tiowing through the shell Vand tube side of theexchanger, respectively, without any interruption in the overall ow ofthese streams through the system.

Corresponding valves and lines are provided for exchanger Zt. Valve 3lmay be closed so that the gas oil charge may pass through valve 31a tothe exchanger inlet through which slurry previously entered. When flowis thus reversed, valve 32 is closed and valve 32a -is opened so thatslurry will enter the shell side of the exchanger to the inlet where gasoil charge previously entered. When the rerouted gas oil reaches valve33 this valve is closed and valve 33a is opened; simultaneously valve 34is closed and valve Ma is opened. The valve connections for exchanger 20are the same as those hereinabove described for exchanger 20, the corresponding valves being indicated by 4an added prime Referring to Figure2, each heat exchanger in this particular case is that previouslyinstalled in a catalytic cracking system 4and comprises a shell 35 about30 inches in diameter provided with upper and lower tube sheets 36,`vhich require about 175 tubes 37, each 11A inches outside diameter and16 feet long. Between the tube sheets are staggered baies 3S for causingsinuous ow of liquid through the shell side of the exchanger from shellinlet 39 to shell outlet 4t?. A transverse bale 41 separates the spacebelow the bottom tube sheet into inlet and outlet zones so that liquidswhich enter the tube side of the exchanger at 42 pass upwardly throughthe right hand bank of tubes, then downwardly through the left hand bankof tubes and out through outlet 43. The valved lines employed inaccordance with our invention are also illustrated in Figure 2 andfurther description thereof is unnecessary in view of the descriptionhereinabove given.

From the foregoing `description it will be apparent that at any time, bymere switching of valves, the gas oil `can be made to ow through thetube side and slurry o-il through the shell side, or vice versa.Furthermore, the flow through the system can be so controlled that thegas oil charge may flow through the shell side of one exchanger land thetube side of the other while the slurry oil is passing through the tubeside of one and the shell side of the other; when operated in thismanner, both the pressure drop between pump 15 and the preheater furnaceand the pressure drop between line 24 and 26 will remain constantregardless of the switching of valves. However, the pressure dropdiiferences in switching the liquids from the shell to the tube side ofthe exchanger are suiiiciently small so that valves may be changed onany particular heat exchanger without materially disturbing flow throughthe remaining heat exchangers. In the example herein described theinvention is applied to an existing heat exchanger and it will beunderstood, of course, that for maximum simplicity of operation, theheat exchanger should be designed to give equal pressure drop throughthe shell sides and tube sides, respectively, for a given stream.

By reversing the valves in the manner hereinabove described, as soon asa significant impairment of heat exchange coefficient is observed thecombined action of the suspended solids and the solvent nature of theslurry oil removes such deposits in a relatively short period of time,which may range from about 2 hours to as much as 6 days. The valves maythen be again reversed to their initial position and the heat exchangersystem operated until impairment of heat transfer coefficient is againindicated at which time this process may be repeated. Thus, without everbypassing any exchanger and, in fact, without taking any exchanger offstream, the heat exchangers may' be operated continuously withsubstantially maximum heat transfer coecient.

The exchanger system hereinabove described was designed to have thefresh feed enter exchangers 18 and 18 at a temperature of about 100 or110 F. and leave exchangers 20 and 20 at atemperature of about 350 F.,the hot slurry entering exchangers 20 Iand Z0 at about 605 F. andleaving exchangers 18 and 18 at about 442 F. As initially operated,these exchangers became so fouled'on the fresh feed side that at the endof the run the fresh feed leaving exchangers 20 and 20 was preheated toonly about 170 F. With fresh feed stripping -in tower 11 it was possibleto maintain an outlet temperature of at least about 250 F. provided thatthe shell sides of the exchanger were periodically cleaned. By operatingin accordance with our invention, it should be possible to maintain theoutlet temperature within about 10 or 15 degrees of the designed figureof 350 F. at all times by reversing the inlet of the streams from shellto tube side and vice versa as soon as the temperature in lines 21 and21 drops by as much, for example, as to 15, e. g. 10 F.

In a test run on a 30,000 bbl./ day uid catalytic cracking unit, one ofthe exchangers (corresponding to 18) was initially operated with freshfeed (containing solids from the Dorr settler) in the shell side andslurry oil in the tube side. In 30 days the corrected overall heattransfer coefficient (Us) gradually dropped from about 38 to 29. Whenflow was transferred so that slurry passed through the shell side andfresh feed through the tubes, said coefficient rose to 33 in a matter ofhours, in 2 days it had reached 37 and in 4 days was up to 40 (i. e. wasmore efficient than at the beginning of the run). During the next 4 daysthe overall heat transfer coeicient gradually dropped back to 36 becauseof deposit formation on the tube side. At that time (8 days from theprevious ow transfer) the ow was again transferred to that originallyemployed, and in a matter of hours the overall heat transfer coetiicientwas back up to 40.

For comparison, other portions of the same charging stock (containingthe same amount of solids per unit volume) and other portions of thesame slurry oil were passed through another heat exchanger with thecharging stock on the shell side throughout the whole test period. Inthis exchanger the initial corrected overall coeiiicient (Us) was about50 and in 20 days it had dropped to 28, in the next 8 days it dropped to27, and at the end of 40 days it was down to 25. This comparative runshows that the use of solids in the charge has some beneficial effect inretarding'accumulation of deposits, but that it fails to keep theexchanger surfaces clean in the manner exhibited by the flow transfertechnique.

An important feature of our invention is that of removing the slightdeposits formed on the fresh feed side of the exchanger by subsequent owof hot slurry therethrough so that the removed deposits are discardedfrom the system with decanted slurry oil and are not carried downstreamto lthe tubes of the preheat furnace. Experience has shown that whenheavy deposits occur on the fresh feed side of the exchangers, there isa tendency for deposits to form downstream of said exchangers, andparticularly in the tubes of the preheat furnace. By removing depositedmaterial with the hot slurry, the preheat furnace tubes are thusprotected so that they operate at increased efficiency for longerperiods of time.

The actual quantity of solids in the slurry oil may vary throughout arelatively Wide range, depending upon the efficiency of the cycloneseparators in the upper part of the reactor of the fluid catalyticcracking system. Usually, the amount of solids carried by the slurry oilis in the range of 0.1 to 1.0 pound per gallon or from about 4 to 42pounds per barrel, such particles being activated 6 clay, syntheticsilica-alumina or silica-magnesia catalysts having a particle size inthe range of about 1 to 100 microns. The net amount of slurrycontinuously produced is passed to a settling zone (e. g. a Dorrsettler) so that as much as possible of the slurry oil may be decantedtherefrom and the catalyst particles may be r'eturned to the system witha minimum amount of such slurry oil. It is known that by introducing thesettled Vsolids to the fresh feed before the feed passes through thepreheat coils, the scouring action of the solids helps to prevent`deposition of coke in said preheat tubes. By introducing the solidsinto the fresh feed before the fresh feed enters the heat exchangesystem a scouring effect may, likewise, be obtained on the fresh feedside of the heat exchangers. However, it is not always desirable tointroduce solids into the fresh feed at this stage of the process andfurthermore, as hereinabove pointed out, the fresh feed does not havethe solvent power for deposits that is exhibited by the hot slurry oiland by ernploying the hot slurry to remove the slight initial depositslaid down by the fresh feed, such deposits are eliminated from thesystem with discarded slurry oil so that they cannot cause coking in thepreheat tubes or form undue amounts of coke on the catalyst in thereactor. By delaying the switching of outlet valves (after inlet valveshave been reversed) until the gas oil has replaced the slurry content ofthat side of the exchanger before reversing the outlet valves of theexchanger, it is possible to minimize any loss of gas oil with decantedslurry oil and also to minimize the entry of any slurry oil into thecharging stock stream; while there may be a slight intermingling of gasoil and slurry oil, this is of no serious consequence since thereversals of inlets occur only periodically at relatively long intervalsso that in overall operations the amount of gas oil lost with slurry oiland the amount of slurry oil contaminating the gas oil charge isrelatively insignificant.

While we have described and shown a manually operated valve system foralternating the flow of uids from shell to tube and tube to shell sidesof the exchanger, it should be understood, of course, that such valvesmay be automatically operated in any known manner in response to apredetermined temperature drop in the charging stock outlet. Pneumatic,electrical and other types of automatic controls are well known to thoseskilled in the art and require no detailed description herein. Also,instead of employing separate alternative valves in each instance, athree-way valve may be employed so that a single valve structure willaccomplish the function of two separate valves. Other modifications andalternative arrangements will be apparent from the above description tothose skilled in the art.

We claim:

1. In a fluid catalytic cracking system including a furnace tube, areactor, a scrubbing tower and at least one heat exchanger comprising avessel, a plurality of tubes therein, and tube headers to which thetubes are secured, the space around the tubes between the headers beingcalled the shell side and the space beyond and within the tubes beingcalled the tube side of said exchanger, wherein charging stock is passedthrough one side of said heat exchanger and thence through the furnacetube to the reactor and eiluent from the reactor is introduced into thescrubbing tower wherein solid catalyst particles are scrubbed out ofpartially condensed product by heavy gas oil components thereof to forma hot slurry which in turn is passed through the other side of said heatexchanger to preheat said charging stock, the charging stock normallypassing through the shell side of the exchanger and the hot slurrythrough the tube side thereof and said exchanger having only one inletline and only one outlet line, respectively, for the shell side and onlyone inlet line and only one outlet line, respectively, for said tubeside, the improvement which comprises a by-pass line from the shellinlet F line to the tube inlet line, a by-pass line from the tube inletline to thershell inlet-line, a by-passline from the shell outlet lineto the tube outlet line, a by-pass line from the tube outlet line to theshell outlet line, and valves in each of said lines so that the ow ofcharging stock and slurry through the tube side and the shell side maybe periodically alternated without changing direction of flow in theexchanger.

2. The system of claim 1 which includes a second exchanger connected inseries with the rst named exchanger, said second exchanger having inlet,outlet and alternate lines and valves corresponding to those of the rstnamed exchanger whereby the liquid charge may always flow through theshell side of one exchanger when it is owing through the tube side ofthe other so that the pressure drop through the exchanger system alwaysremains substantially constant.

3. The system of claim 1 which includes a stripping tower, a chargingstockv inlet leading tothe upper part of said tower, a fuel gas inlet atan intermediate part of said tower whereby liquid may be accumulated inthe lower part thereof, a gas outlet at the top of said tower, av pump,and connections for pumping accumulated stripped charging stock from thelower part of said tower to saidrheat exchanger.

References Cited in the le of this patent UNITED STATES PATENTS1,006,197 Frasch Oct. 17, 1911 2,396,109 Martin Mar. 5, 1946 2,490,750Grewin et al. Dec. 6, 1949 2,490,759 Tyden Dec. 6, 1949 2,493,494 MartinJan. 3, 1950 2,57 6,843 Lockman Nov. 27, 1951 2,647,570 Lockman Aug. 4,1953 2,723,948 McCurdy Nov. 15, 1955

1. IN A FLUID CATALYTIC CRACKING SYSTEM INCLUDING A FURNACE TUBE, AREACTOR, A SCRUBBING TOWER AND AT LEAST ONE HEAT EXCHANGER COMPRISING AVESSEL, A PLURALITY OF TUBEES THEREIN, AND TUBE HEADERS TO WHICH THETUBES ARE SECURED, THE SPACE AROUND THE TUBES BETWEEN THE HEADERS BEINGCALLED THE "SHELL SIDE" AND THE SPACE BEYOND AND WITHIN THE TUBES BEINGCALLED THE "TUBE SIDE" OF SAID EXCHANGER, WHEREIN CHARGING STOCK ISPASSED THROUGH ONE SIDE OF SAID HEAT EXCHANGER AND THENCE THROUGH THEFURNACE TUBE TO THE REACTOR AND EFFLUENT FROM THE REACTOR IS INTRODUCEDINTO THE SCRUBBING ROWER WHEREIN SOLID CATALYST PARTICLES ARE SCRUBBEDOUT OF PARTIALLY CONDENSED PRODUCT BY HEAVY GAS OIL COMPONENTS THEREOFTO FORM A HOT SLURRY WHICH IN TURN IS PASSED THROUGH THE OTHER SIDE OFSAID HEAT EXCHANGER TO PREHEAT SAID CHARGING STOCK, THE CHARGING STOCKNORMALLY PASSING THROUGH THE SHELL SIDE OF THE EXCHANGER AND THE HOTSLURRY THROUGH THE TUBES SIDE THEREOF AND SAID EXCHANGER HAVING ONLY ONEINLET LINE AND ONLY ONE OUTLET LINE, RESPECTIVELY, FOR THE SHELL SIDEAND ONLY ONE INLET LINE AND ONLY ONE OUTLET LINE, RESPECTIVELY, FOR SAIDTUBE SIDE, THE IMPROVEMENT WHICH COMPRISES A BY-PASS LINE FROM THE SHELLINLET LINE TO THE TUBE INLET LINE, A BY-PASS LINE FROM THE TUBE INLETLINE TO THE SHELL INLET LINE, A BY-PASS LINE FROM THE SHELL OUTLET LINETO THE TUBE OUTLET LINE, A BY-PASS LINE FROM THE TUBE OUTLET LINE TO THESHELL OUTLET LINE, AND VALVES IN EACH OF SAID LINES SO THAT THE FLOW OFCHARGING STOCK AND SLURRY THROUGH THE TUBE SIDE AND THE SHELL SIDE MAYBE PERIODICALLY ALTERNATED WITHOUT CHANGING DIRECTION OF FLOW IN THEEXCHANGER.